I tried to install Skype by downloading the .deb file from their website.
The installation aborted due to unmet dependencies and Skype was not installed by Ubuntu Software Center. From then on, I couldn't install any packages and none of the package management utilities worked: they all complained of unmet dependencies.

The formatting in your question is badly broken, and cannot easily be fixed. If you paste the text from the Terminal into your question again, you can select it and use the <$> tool to format it properly. Blockquotes should not be used for code (unless you're quoting it from another source, in which case the code should typically be in a code block which itself is enclosed in a blockquote).
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Eliah KaganJul 7 '12 at 12:32